1. Field of the Invention
The present invention relates to an image-signal processing unit that carries out signal processing on an image signal generated by capturing an optical image in which the amount of a specific primary color is far greater than the other primary colors, such as in an optical image captured by an electronic endoscope.
2. Description of the Related Art
An imaging device generates an image signal corresponding to a captured optical image of a subject. Predetermined signal processing is carried out on the generated image signal so that the colors in a subject image are displayed as accurately as possible. After the image signal undergoes predetermined signal processing, the rendered image is displayed on a monitor.
Imaging devices may be mounted in various apparatuses. For example, an electronic endoscope which comprises an imaging device at the head end of an insertion tube is known. A medical electronic endoscope is used to observe the inside of the human body. CCD imaging devices with complementary color filters are generally used in such medical electronic endoscopes.
A CCD imaging device with complementary color filters generates complementary-color signal components, such as Mg, Ye, G, and Cy. In the signal processing for a general medical electronic endoscope, a wide-band luminance signal and a narrow-band luminance signal are generated based on the complementary color signal. In addition, chrominance difference signals are generated based on the narrow-band luminance signal. The wide-band luminance signal and the chrominance difference signals are transmitted to a monitor. The monitor generates primary color signal components, such as R, G, and B, based on the received wide-band luminance signal and the chrominance difference signals. An image based on the primary color signal components is displayed on the monitor. Incidentally, the primary color signal components can be generated in the endoscope processor and transmitted to the monitor.
The main subject of a medical electronic endoscope is the inside of an organ, whose optical image is mostly reddish. When a reddish optical image is captured, it is possible for the signal level of just the red signal component (which is generated based on the wide-band luminance signal) to exceed the upper limits of the endoscope processor or a monitor. If the signal level of only the red signal component exceeds the upper limit, colors cannot be accurately displayed on a monitor.